linux-kernel - RE: [PATCH v2] Drivers: hv: vmbus: fix the race when querying & updating the percpu list

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Date:	Fri, 20 May 2016 15:30:20 +0000
From:	Dexuan Cui <decui@...rosoft.com>
To:	Dexuan Cui <decui@...rosoft.com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"driverdev-devel@...uxdriverproject.org" 
	<driverdev-devel@...uxdriverproject.org>,
	"olaf@...fle.de" <olaf@...fle.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"jasowang@...hat.com" <jasowang@...hat.com>,
	KY Srinivasan <kys@...rosoft.com>,
	"vkuznets@...hat.com" <vkuznets@...hat.com>,
	Rolf Neugebauer <rolf.neugebauer@...ker.com>,
	Dave Scott <dave.scott@...ker.com>
CC:	Haiyang Zhang <haiyangz@...rosoft.com>
Subject: RE: [PATCH v2] Drivers: hv: vmbus: fix the race when querying &
 updating the percpu list

> From: devel [mailto:driverdev-devel-bounces@...uxdriverproject.org] On
> Behalf Of Dexuan Cui
> Sent: Wednesday, May 18, 2016 11:44
> To: gregkh@...uxfoundation.org; linux-kernel@...r.kernel.org; driverdev-
> devel@...uxdriverproject.org; olaf@...fle.de; apw@...onical.com;
> jasowang@...hat.com; KY Srinivasan <kys@...rosoft.com>;
> vkuznets@...hat.com
> Cc: Haiyang Zhang <haiyangz@...rosoft.com>
> Subject: [PATCH v2] Drivers: hv: vmbus: fix the race when querying &
> updating the percpu list
> 
> There is a rare race when we remove an entry from the global list
> hv_context.percpu_list[cpu] in hv_process_channel_removal() ->
> percpu_channel_deq() -> list_del(): at this time, if vmbus_on_event() ->
> process_chn_event() -> pcpu_relid2channel() is trying to query the list,
> we can get the general protection fault:
> 
> general protection fault: 0000 [#1] SMP
> ...
> RIP: 0010:[<ffffffff81461b6b>]  [<ffffffff81461b6b>]
> vmbus_on_event+0xc4/0x149
> 
> Similarly, we also have the issue in the code path: vmbus_process_offer() ->
> percpu_channel_enq().
> 
> We can resolve the issue by disabling the tasklet when updating the list.
> 
> Reported-by: Rolf Neugebauer <rolf.neugebauer@...ker.com>
> Cc: Vitaly Kuznetsov <vkuznets@...hat.com>
> Signed-off-by: Dexuan Cui <decui@...rosoft.com>
> ---
> 
> v2: added tasklet_schedule() after tasklet_enable(). Thanks, Vitaly!

Please ignore the patch for now.

I found an issue with the patch:  after I moved percpu_channel_deq()
from  hv_process_channel_removal() to vmbus_close_internal(), the
channel couldn't be removed from the per-cpu list, if the channel state
was not CHANNEL_OPENED_STATE.

I'll have to think about this and fix the issue in the next version.

Thanks,
-- Dexuan